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To what extent can a worker's perception of exertion be used to establish what is an acceptable level of work? There is an increasing body of knowledge that suggests that safe levels of work can be established using psychophysical and psycho-scaling tools to assess the worker's perception of exertion. The purpose of this chapter is to review laboratory methods used to assess the perceived exertion of workers, examine the role of task variables that affect perceived exertion (i.e., force, work rate, and type of work-static versus dynamic) and assess the potential of psychological methods for achieving the goal of establishing safe levels of work. Numerous studies over the last 10 years have investigated the occupational causes of upper extremity musculoskeletal disorders. Armstrong and associates 1 recently reviewed a series of epidemiologic studies that linked various job or task attributes with disorders affecting the musculoskeletal system. To date, however, relatively few studies have been conducted that provide information which defines the limits of acceptable or safe manual work for the majority of healthy workers. One reason may be the manner in which hun1an work capacity is assessed. To assess work capacity and the potential for musculoskeletal injury, investigators rely on one or two approaches: If the work is heavy, but intermittent, biomechanical measures of strength may serve as indicators of work capacity. If the work is light, but continuous, physiological measures of endurance may be needed. Studies of muscle endurance, for example, provide useful information on the strength of an individual muscle or even a group of muscles. However, the problem is that even simple manual work activities can require the use of numerous muscles, joints, ligaments, and tendons in varying degrees. Information on the functioning of one or more muscles cannot provide much predictive insight as to the level of exertion a worker is willing to experience to carry out the job safety.